Prostate cancer is the most common non-cutaneous malignancy in men and the second leading cause of death in men from cancer in the western world (Jemal A, Siegel R, Xu J, Ward E. Cancer Statistics. Cancer J Clin 2010; 60:277-300). As a male sexual organ, development of the prostate is highly regulated by androgens, the AR and by the products of androgen dependent genes. During all stages of prostate cancer progression, the disease remains dependent upon androgens. Anti-androgens, including AR antagonists, are used therapeutically to reverse the dependence of the tumor upon the actions of androgen (Scher H, Sawyers C. Biology of progressive, castration-resistant prostate cancer: directed therapies targeting the androgen-receptor signaling axis. J Clin Oncol 2005; 23:8253-8261; Tran C, Ouk S, Clegg N, Chen Y, Watson P, Arora V, et al. Development of a second-generation antiandrogen for treatment of advanced prostate cancer. Science 2009; 324:787-790; Scher H, Fizazi K, Saad F, Taplin M, Sternberg C, Miller K, et al. Increased survival with enzalutamide in prostate cancer after chemotherapy. N Engl J Med 2012; 367:1187-1197). Unfortunately, the efficacy of even second-generation, highly potent AR antagonists, such as MDV-3100 (enzalutamide, Xtandi®), is short-lived in many patients.
AR antagonists have transformed patient care by targeting a key nodal point in tumor cell signaling. However, as with other molecularly targeted cancer therapies across different oncology indications, the emergence of acquired resistance via mutation of the therapeutic target is not uncommon. This is best exemplified by imatinib-treated patients with chronic myeloid leukemia in whom ABL kinase mutations render leukemia cells resistant to imatinib. Multiple next-generation ABL inhibitors have since been developed to circumvent the mutation and with activity in this setting (Gorre M, Mohammed M, Ellwood K, Hsu N, Paquette R, Rao P, Sawyers C. Clinical resistance to STI-571 cancer therapy caused by BCRABL gene mutation or amplification. Science 2001; 293:876-80; O'Hare T, Deininger M W, Eide C A, Clackson T, Druker B J. Targeting the BCR-ABL signaling pathway in therapy-resistant Philadelphia chromosome-positive leukemia. Clin Cancer Res 2011. 17: 212-21).
Importantly, the activity of second- and third-generation AR inhibitors indicates that the disease remains “addicted” to a deregulated driver. This has led to the paradigm of sequential therapy targeting the same driver oncogene in distinct resistant states and is applicable herein to targeting of AR and the lineage dependence of AR signaling.
AR mutations that result in receptor promiscuity and the ability of these anti-androgens to exhibit agonist activity might at least partially account for this phenomenon. For example, hydroxyflutamide and bicalutamide act as AR agonists in T877A and W741L/W741C AR mutants, respectively.
In the setting of prostate cancer cells that were rendered castration resistant via overexpression of AR, it has been demonstrated that certain anti-androgen compounds, such as bicalutamide, have a mixed antagonist/agonist profile (Tran C, Ouk S, Clegg N, Chen Y, Watson P, Arora V, et al. Development of a second-generation antiandrogen for treatment of advanced prostate cancer. Science 2009; 324:787-790). This agonist activity helps to explain a clinical observation, called the anti-androgen withdrawal syndrome, whereby about 30% of men who progress on AR antagonists experience a decrease in serum PSA when therapy is discontinued (Scher, H. I. and Kelly, W. K., J Urol 1993 March; 149(3): 607-9). Prostate specific antigen decline after antiandrogen withdrawal: the flutamide withdrawal syndrome.
Accumulating evidence indicates that castration-resistant prostate cancer (CRPC) remains dependent upon AR signaling through reactivation of AR signaling (Yuan X, Balk S. Mechanisms mediating androgen receptor reactivation after castration. Urol Oncol 2009; 27: 36-41; Linja M, Savinainen K, Saramaki O, Tammela T, Vessella R, Visakorpi T. Amplification and overexpression of androgen receptor gene in hormone-refractory prostate cancer. Cancer Res 2001; 61:3550-; Chen C, Welsbie D, Tran C, Baek S, Chen R, Vessella R, Rosenfeld M, Sawyers C. Molecular determinants of resistance to antiandrogen therapy. Nat Med 2004; 10(1): 33-9.555). Point mutation in the ligand-binding domain (LBD) of AR accounts for 10-20% of resistance and is characterized by receptor activation, rather than inhibition, by anti-androgen drugs (Beltran H, Yelensky R, Frampton G, Park K, Downing S, MacDonald T, et al. Targeted next-generation sequencing of advanced prostate cancer identifies potential therapeutic targets and disease heterogeneity. Eur Urol 2013; 63(5): 920-6; Bergerat J, Ceraline J. Pleiotropic functional properties of androgen receptor mutants in prostate cancer. Hum Mutat 2009; 30(2): 145-57). Many of these mutations broaden ligand specificity, and some confer resistance by converting the AR antagonist into an agonist of the mutant receptor (Veldscholte J, Ris-Stalpers C, Kuiper G G, Jenster G, Berrevoets C, Claassen E, van Rooij H C, Trapman J, Brinkmann A O, Mulder E. A mutation in the ligand binding domain of the androgen receptor of human LNCaP cells affects steroid binding characteristics and response to anti-androgens. Biochem Biophys Res Commun. 1990; 173: 534-40; Haapala K, Hyytinen E, Roiha M, Laurila M, Rantala I, Helin H, Koivisto P. Androgen receptor alterations in prostate cancer relapsed during a combined androgen blockade by orchiectomy and bicalutamide. Lab Invest 2001; 81(12):1647-1651; Hara T, Miyazaki J, Araki H, Yamaoka M, Kanzaki N, Kusaka M, Miyamoto M. Novel mutations of androgen receptor: a possible mechanism of bicalutamide withdrawal syndrome. Cancer Res 2003; 63(1):149-153).
One mutation, phenylalanine to leucine at position 876 (F876L) of AR, was recently shown to arise in response to MDV-3100 and ARN-509 in preclinical models and in patients undergoing therapy with ARN-509 (Clegg N, Wongvipat J, Joseph J, Tran C, Ouk S, Dilhas A, et al. ARN-509: a novel antiandrogen for prostate cancer treatment. Cancer Res 2012; 72(6): 1494-503; Balbas M, Evans M, Hosfield D, Wongvipat J, Arora V, Watson P, et al. Overcoming mutation-based resistance to antiandrogens with rational drug design. Elife 2013. 2: e00499; Korpal M, Korn J, Gao X, Rakiec D, Ruddy D, Doshi S, et al. An F876L mutation in androgen receptor confers genetic and phenotypic resistance to MDV3100 (enzalutamide). Cancer Discov 2013; 39:1030-1043; Joseph J D, Lu N, Qian J, Sensintaffar J, Shao G, Brigham D, Moon M, Maneval E C, Chen I, Darimont B, Hager J H. A clinically relevant androgen receptor mutation confers resistance to second-generation antiandrogens enzalutamide and ARN-509. Cancer Discov 2013; 3:1020-1029).
AR F876L confers resistance to MDV-3100 and ARN-509. Comprehensive biological studies have demonstrated that prostate cancer cells harboring this mutation continued to grow when treated with either compound. In vitro reporter assays confirmed resistance and demonstrate agonist conversion of both compounds and in tumors engineered to express AR F876L, neither compound controlled tumor growth. Furthermore, the AR F876L mutant is detected in ARN-509-treated patients with progressive CRPC. The mutation was detected in the plasma DNA of patients undergoing longitudinal analysis in 3 of 29 patients eligible for assessment. All 3 of the patients were amongst the 18 patients with an increase in prostate specific antigen (PSA) whilst on drug, indicative of disease progression (Joseph 2013).
Structural modeling of wild-type (WT) and F876L mutated AR bound with MDV-3100, indicated that helices 11 and 12 were differentially displaced. Within the LBD of AR in the F876L mutant, helix 12 is not displaced by MDV-3100 as it is in WT AR, and this allows MDV 3100 to function as an agonist. The compounds described herein are designed to act as antagonists (third-generation), where second-generation compounds are not active.
Thus, androgen receptor antagonists of the present invention may provide therapeutic benefit for the treatment of prostate cancer and other diseases, syndromes, disorders, or conditions associated with androgen-resistant ARs or an AR mutant associated with castration-resistant prostate cancer.